Method of producing a ferrous sintered article



States Patent SernNo. 474,185 a ,7

Int. Cl. C22c 33/02 U.S.:Cl. 75-201 5 Claims This invention deals generally with an improved method for manufacturing sintered ferrous workpieces, and deals more particularly with a method for producing ferrous workpieces from powdered or granular ferrous material by compaction and sintering of said material, with the incorporation of small amounts of elemental sulfur and elemental copper in synergistic quantities therein to im prove strength characteristics of the finished article and improve the efiiciency of the manufacturing process therefor.

This application isa continuation-in-part of co-pending US. application Ser. No. 329,230, filed Dec. 9, 1963, now abandoned.

A levelof strength obtainable from iron powder, without the introduction of other alloying metals (such as nickel or chromium), is seriously limited in practice to values not substantially exceeding 100,000 p.s.i. on the standard fiber stress test (ASTM B312-58T). To attain this strength it is usually necessary in commercial practice to compress the powder, containing 1.2-1.5% graphite and 1% lubricant at about 50-60,000 p.s.i., and then to heat in a sintering furnace usually for not less than 30 minutes at 2050 F. 1 (or for about 45 minutes including heat-.up time), in an atmosphere which is conventionally endo or exo gas, or, less commonly, dissociated ammonia in order to maintain a substantially non-oxidizing atmosphere.

More strenuous conditions, e.g. higher compaction pressures higher sinter temperatures, longer retention times, are sometimes employed but the gains in strength resulting therefrom are generally small, and the increase in cost of the operations is sufliciently great, that the industry has not found these lines of development particularly fruitful, although some variations in all these conditions are to .be found in current practice.

[In general, when it is necessary to obtain higher strength, alloying metals may be added (e.g., copper), or a variety of densifying and infiltrating techniques emp1oyed.1*These techniques have the effect of increasing density. as well as strength, diminishing porosity, and in general, appreciably increasing cost.

Although it has been proposed that elemental sulfur provides greatly improved strength at relatively low compact ign pressures, and that copper is compatible with sulfur in this respect, it is believed that no one has recognized heretofore that in combinations involving less than 1% each, respectively, copper and sulfur, with powdered iron sintered articles there is thus provided an unexpected, greatly. improved effect unpredictable from the activity of either sulfur or copper alone. Although copper has been used for various reasons in powdered iron mixes, it has :always been used in quantities in excess of one percent. While it is true copper has been disclosed in ranges of 0 to 5% for example, the import is clear that if copper is to be of any use whatever, it should be used in excess of 1%, otherwise it is just as well omitted entirely, or used at 0%.

It is therefore an object of this invention to provide a method for producing an improved metallic article.

It is also an object of this invention to provide a method of producing an improved ferrous article by compaction of powdered ferrous metal particles into a predetermined shape and sintering thereof.

Another object of this invention is to provide a sintered ferrous metal workpiece having improved properties of strength and hardness by incorporating into the presinter mixture thereof minute quantities of elemental sulfur and copper.

Other related objects will appear from the following disclosure and claims.

I have found that in the conventional method of compaeting powdered ferrous metal material under high pressures into a predetermined shape, followed by sintering such shape to develop internal strength, the end product can be greatly improved by the addition of a quantity of elemental sulfur in amounts less than 1%, based on the amount of iron present, in combination with a quantity of elemental copper in amounts less tha 1%, based upon the weight of iron present, said sulfur and copper being added to the precompaction mix in the form of discrete particles.

This discovery specifically excludes utilization of metal sulfides in combination with copper, as only elemental sulfur is workable to achieve the results hereinafter described.

Further, the sulfur of my invention may not be relied upon to function in the manner I desire if it is included in any way as a alloy intimately dispersed within the structure of the ferrous material which forms the bulk. of the body of the compacted article.

Thus, in my precompaction mix, both the elemental sulfur and elemental copper must be present in their uncombined forms, and physically dispersed throughout the precompaction, powdered iron mix.

Proceeding to the description of a preferred embodiment of my invention, it is desirable, using conventional methods, to effect the initial iron-to-iron bond by compaction.

As a typical example, the conventional additives of powdered iron, lubricant, and graphite were intimately admixed with elemental sulfur and elemental copper by tumbling. A thirty minute mixing period was employed. although shorter periods will suffice.

The following basic formulation was used:

Parts Powdered iron Graphite 1.4 Zinc stearate (lubricant) 0.75 Elemental sulfur Variable Elemental copper Variable The various mixtures were compacted in this series of tests at 32 tons per sq. in. to form standard ASTM samples 1%" x A" x /2".

The bodies were then heated in a tube furnace, with a preheating period maintained at approximately 600 F. for 15 minutes to simulate heat-up time in large scale operations. The samples were then held in the sintering zone maintained at a temperature of 205 0 F. for a period of 20 minutes. This series of tests was run in an atmosphere of simulated exo gas which would typically analyze at 10% CO, 20% H with the balance nitrogen. Commercial exo" gas may also contain traces of methane, CO oxygen and moisture.

Table I below illustrates the transverse rupture strengths of a series of 15 samples containing varying amounts of elemental sulfur and copper.

From the foregoing table it will be seen that at zero parts copper and sulfur, rupture strength is relatively low. Maintaining copper at zero and running the elemental sulfur up to 0.4 part, the improvement based solely on sulfur alone is fairly substantial; "while maintaining sulfur at zero and running copper up to 1.2, strength is surprisingly improved, considering no prior investigator had explored copper below one percent.

However, as soon as minor quantities of elemental sulfur and copper both are introduced, it will be seen that at 0.2 sulfur and 0.3 and 0.6 copper, respectively, and at 0.4 sulfur and 0.6 copper, the result is far more than double the strength achieved by either copper or sulfur when present alone without the other. Although at 0.2 and 0.4 sulfur, and 1.2% copper, there was some slight increase in the strength over, respectively, 0.2 and 0.4 sulfur at 0.9% copper, the increase was not appreciably great to indicate that the peak effect would become greater at higher concentrations of either.

The corresponding hardness of the samples of Table I is set forth below in Table II.

From the foregoing table it will be apparent that, in the absence of sulfur, the increase in copper shows a straight increase in strength for all levels of copper studied. And when sulfur is present, there is a relatively large increase in strength attributable solely to sulfur at copper.

At 0.2 and 0.4 part sulfur, there appears to be a low concentration of copper at about 0.30.6 part, at which maximum values of strength are observed. Further small increases in copper result in actual decreases in strength as will be seen, and large increases in copper (into the range beyond 1% copper) convert the system into one in which sulfur effects are lost, and which the conventional copper response is observed, thus tying in with observations of prior investigators that, (absent sulfur) copper to be used only in excess of one percent if its benefits are to be realized; Russo, US. Pat. No. 3,126,699, observing that if sulfur were used, copper tested in excess of one percent provided no added strength.

It will further be seen that the hardness values roughly parallel the strength values.

In further tests, in which the compaction pressure was reduced to 25 tons per sq. in., with 1% zinc stearate being used as a lubricant, the base formulation noted above being maintained constant throughout with the exception of variation in sulfur and copper, the following data were observed:

In Table HI, the peculiar loss of strength with increasing copper is again observed.

In combination with my novel invention, any number of well-known lubricants may be utilized for expediting the compaction step, such as metal soaps, fatty acids, various derivatives of the fatty acids, etc.; such lubricants are fully compatible with my invention, and may be incorporated into the powdered metal mix, or applied to the surface of the die wall, etc. Varying compaction pressures as low as 20 tons per sq. in. or as high as 4050 tons per sq. in. may be used with variable graphite from 0.8 to 1.6 parts/ parts Fe. Furnace conditions and sintering temperatures may be varied, with 1950 F. being regarded as about the lowest practical sintering temperature. I

In summary, my discovery permits the attainment of unexpectedly high strengths by using relatively small amounts of elemental copper in quantities less than 1 part,

in the presence of relatively small amounts of sulfur in 7 amounts less than 1 part, parts expressed in terms of parts by weight of iron used.

Strengths achieved, and as typified by the foregoing examples, are attainable in a sulfur-free system only at concentrations of copper normally equal to or exceeding 2 parts copper, which constitutes quite an advance in the art since copper is much more expensive than elemental sulfur, and, if equal strength can be achieved at half the normal addition of copper, there is quite an obvious economic gain.

I have found that the purer forms of powdered iron are generally preferred for producing stronger compacted sintered articles than are iron powders covered by oxide layers. Thus, commercial hydrogen reduced sponge iron is superior to iron powders which are either insufiiciently reduced, or which are subsequently reoxidized. Thus, powdered iron consisting of at least 95% Fe is desirable, with powdered iron analyzing at least 98% Fe being preferable.

I believe some unexpected, as yet unexplained, reaction takes place in the system containing elemental sulfur and elemental copper described above at concentrations greater than 0.1 and less than 1.0 part each, respectively, sulfur and copper, the preferred range of each being found in an amount greater than 0 part, but less than 0.5 part elemental sulfur, with about 0.3 or more, but less than 0.9 part elemental copper, as exemplified in Table I above.

The preferred particle size of my sulfur and copper is preferably not coarser than that of the iron powder with which it is to be used and in general the sulfur and copper would be approximately 80 mesh and preferably mesh. However, it will be apparent that under certain conditions, particle size outside these limits would still be conducive to achieving the beneficial results of my invention.

Having thus described my invention, I claim:

1. The method of producing a sintered ferrous article comprising the steps of:

(a) thoroughly admixing from about 0.1% to about 0.5% elemental sulfur and about 0.1% to about 0.7% copper, with powdered iron particles, said percentages of sulfur and copper based on the weight of said iron, prior to sintering,

(b) compacting said admixture into a coherent shape at at least 50,000 p.s.i., and sintering, in a substantially nonoxidizin atmosphere, said compacted shape,

to provide a ferrous article having improved strength.

2. The method of producing a sintered ferrous article comprising the steps of:

(a) thoroughly admixing from about 0.1% to about 0.5% elemental sulfur and about 0.1% to about 0.7% copper, with powdered iron particles, said percentages of sulfur and copper based on the weight of said iron, prior to sintering,

(b) compacting said admixture into a coherent shape at at least 50,000 p.s.i.,

(c) sintering said compacted admixture at at least 1950 F. in a substantially nonoxidizing atmosphere,

to. provide a ferrous article having improved strength.

3. The method of producing a sintered ferrous article comprising the steps of:

(a) thoroughly admixing from about 0.1% to about 0.5% elemental sulfur, and 0.1% to about 0.7% copper, with substantially pure powdered iron particles comprising at least 95% Fe, said percentages of sulfur andcopper based upon the Weight of said iron, prior to sintering, said iron particles constituting at least 95 of said mix,

(b) compacting said admixture into a coherent shape,

(c). sintering said shape in a substantially nonoxidizing atmosphere at at least 195 0 F.,

to provide a ferrous article having improved strength.

4. The method of producing a sintered ferrous article comprising the steps of:

(a) thoroughly admixing from about 0.1% to about 0.5% elemental sulfur, about 0.1% to about 0.7% copper, and not more than 2.5% graphite, with substantially oxide-free powdered ferrous particles, said particles consisting of substantially 95% Fe, said percentages of sulfur, copper and graphite based on 100 parts by Weight of said iron, prior to sintering, said ferrous particles constituting at least 95% by Weight of said mix, (b) compacting said admixture into a coherent shape, (c) sintering said shape in a substantially nonoxodizing atmosphere at at least 1950 F., to provide a ferrous article having improved strength.

5. A sintered reaction product of an intimate powdered iron composition admixture, the major portion thereof consisting essentially of, prior to sintering, 100 parts by weight of powdered iron, about 0.1 to about 0.5 part elemental sulfur and about 0.1 to about 0.7 part copper,

said sulfur and copper being present in the form of discrete particles.

References Cited UNITED STATES PATENTS 3,120,699 2/1964 Russo 29-1825 CARL D. QUARFORTH, Primary Examiner A. J. STEINER, Assistant Examiner US. Cl. X.R. 75--21l, 214 

1. THE METHOD OF PRODUCING A SINTERED FERROUS ARTICLE COMPRISING THE STEPS OF: (A) THOROUGHLY ADMIXING FROM ABOUT 0.1% TO ABOUT 0.5% ELEMENTAL SULFUR AND ABOUT 0.1% TO ABOUT 0.7% COPPER, WITH POWDERED IRON PARTICLES, SAID PERCENTAGES OF SULFUR AND COPPER BASED ON THE WEIGHT OF SAID IRON, PRIOR TO SINTERING, (B) COMPACTING SAID ADMIXTURE INTO A COHERENT SHAPE AT AT LEAST 50,000 P.S.I., AND SINTERING, IN A SUBSTANTIALLY NONXOIDIZING ATMOSPHERE, SAID COMPACTED SHAPE, TO PROVIDE A FERROUS ARTICLE HAVING IMPROVED STRENGTH. 